1. Field of the Invention
The present invention relates to a planar display apparatus utilizing an electron beam.
2. Description of the Related Art
FIG. 18 is a perspective view of a part of a conventional planar display apparatus described in, for example, Japanese Patent Laid-Open No. 184239/1988. In FIG. 18, the reference numeral 1 represents a linear hot cathode as an electron radiation source which emits electrons when electric conduction is established, the linear hot cathode 1 being connected to a holder (not shown). The reference numeral 2 denotes a mesh electrode having an oval cross-section and a multiplicity of small holes 3 for passing electrons therethrough. By applying an appropriate potential to the mesh electrode 2, electrons are taken out of the linear hot cathode 1. The reference numeral 4 represents a front glass (display screen) with the inside surface coated with dot-like three kinds of dot-like phosphor materials 5 which emit red, green and blue lights when excited by the electrons drawn out by the mesh electrode 2. On the fluorescent substances 5, an aluminum film (not shown) is provided for imparting conductivity. By applying a voltage of about 10 to 30 KV to the aluminum film, the electrons are accelerated and excite the fluorescent substances 5 so as to emit light.
The referential numeral 6 represents a control electrode portion disposed between the front glass 4 and the linear hot cathode 1 in close proximity thereto so as to allow or obstruct the passage of the electrons which are taken out by the mesh electrode 3 and directed toward the front glass 4. As shown in the exploded view of the structure of the control electrode portion 6 in FIG. 19, the control electrode portion 6 is composed of an insulating substrate 8 having electron-passing holes 7 which correspond to the picture elements on the front glass 4, a first control electrode group 9 provided on the undersurface of the insulating substrate 8 and a second control electrode group 10 provided on the upper surface of the insulating substrate 8. The first control electrode group 9 is composed of a plurality of strip metal electrodes 9a. The metal electrode 9a is provided with electron passing portions 9b which correspond to the respective picture elements in one row. Similarly, the second control electrode group 10 is composed of a plurality of strip metal electrodes 10a. The metal electrode 10a are provided with electron passing portions 10b which correspond to the respective picture elements in one vertical line.
Each of the electron passing portions 9b, as well as the electron passing portions 10b, is a reticulate portion produced by making a multiplicity of small holes 11 in the metal electrodes 9a (10a) at the portion corresponding to each of the electron-passing holes 7 in the insulating plate 8, as shown in an enlarged view of FIG. 20.
The periphery of the front glass 4 extends downward in a curved state and is closed (not shown) below a rear electrode 12. The interior of the front glass 4 is maintained at a vacuum. Each electrode in the sealed glass container is electrically connected to the external elements from the sealing portion provided on the side surface.
The operation of the conventional planar display apparatus will now be explained. Electrons are drawn out of the linear hot cathode 1 by the porous cover electrodes 2. The electrons are attracted to the first control electrode group 9 and reach the control electron portion 6.
The voltage applied to each electrode will here be explained on the assumption that the average voltage applied to the linear hot cathode 1 is 0 V as a reference voltage. To the mesh electrode 2, a voltage about 5 to 30 V higher than the voltage applied to the linear hot cathode 1 is applied. To the metal electrode 9a of the first control electrode group 9, a positive potential about 20 to 40 V higher than the potential applied to the linear hot cathode 1 is applied. This voltage is only applied to one metal electrode 9a of the first electrode group 9 at a time, which are arranged orthogonally to the linear hot cathode 1.
The electron current density on the front surface of the metal electrode 9a is preferably substantially uniform. It is possible to make the electron current density uniform by controlling the oval cylinder shape of the mesh electrode 2, the position of the first control electrode group 9 and the voltage applied to each metal electrode 9a.
The operation of the control electrode portion 6 is not described in Japanese Patent Laid-Open No. 184239/1988 but described in, for example, Japanese Patent Laid-Open Nos. 172642/1987 and 126688/1989. In the general matrix type display described in these documents, the operation of the control electrode portion 6 is as follows. As described above, only one metal electrode 9a in the first control electrode group 9 becomes a positive potential and the other metal electrodes 9a have 0 V or a negative potential. In this case, the electrons emitted from the linear hot cathode 1 are attracted only to this one metal electrode 9a having a positive potential. The electrons pass through the electron passing portions 9b of the metal electrode 9a and enter the respective electron-passing holes 7 of the insulating substrate 8. All the electrons which have entered the electron-passing holes 7 do not reach the front glass 4. In other words, of the second control electrode group 10 disposed above the electron-passing holes 7, the electrons pass only through the electron passing portions 10b of the metal electrode 10a to which a potential of, for example, 40 to 100 V is applied and do not pass through the electron passing portions 10b of the other metal electrodes 10a which have 0 V or a negative potential. The electrons at these portions stay in the electron-passing holes 7. Consequently, the electrons pass only through the electron-passing hole 7 at the intersection of the one metal electrode 9a of the first control electrode group 9 to which a positive potential is applied so as to turn it on and the metal electrode 10a of the second control electrode group 10 to which a positive potential is applied. The electrons which have thus passed through the electron-passing hole 7 cause the fluorescent substance 5, at the position of the picture element which corresponds to the electron-passing hole 7, to emit light for displaying a picture on the screen. Therefore, by so controlling the application of the potential to each of the metal electrodes 9a and 10a that the intersection corresponds to a desired light emitting position, a desired picture display is realized. For example, a picture is displayed by consecutively scanning and turning on the metal electrodes 9a of the first control electrode group 9 one by one and, synchronously therewith, consecutively turning on the metal electrodes 10a of the second control electrode group 10 which correspond to the respective light emitting positions. This scanning operation is repeated for a period which is imperceptible to the human eyes, for example, 60 frames per second.
The electron passing portions 9b and 10b, which are reticulate portions produced by making a multiplicity of small holes 11 in the metal electrodes 9a and 10a, respectively, as explained above with reference to FIG. 20, are so designed as to obstruct the passage of electrons when 0 V or a negative potential of several 10 V is applied to each of the control electrodes 9 and 10.
The luminance of each picture element is controlled by the time for which each metal electrode 10a of the second control electrode group 10 is on. If it is assumed that the time for which the first control electrode group 9 is on is T.sub.1 and if the luminance of the picture element at a predetermined position is intended to be P%, the time for which the metal electrode 10a of the second control electrode group 10 which corresponds to that position is on is set at P.T.sub.2 /100.
In such a conventional planar display apparatus, each of the first control electrode group 9 and the second control electrode group 10 must be composed of strip electrodes arranged in each row and each vertical line, respectively. Use of such a strip electrode is disadvantageous because there is a limitation in finer and more accurate displaying function of a planar display apparatus due to the limitation in the accuracy in processing the strip electrodes.
There is also a great trouble in the manufacture of the strip electrodes such as difficulty of fixing and holding them separately from each other.
In addition, since the electron passing portion has a reticulate structure provided with a multiplicity of small holes, electrons hit against the reticulate portion when they pass through the electron passing portion and the lowering of the electron passing ratio, which may lead to the reduction in the luminance of the planar display apparatus, is inevitable.
As to the luminance, electrons are gradually attached to the portions of the surface of the insulating substrate 8 which are not covered with the metal electrodes of the first and second control electrode groups, and the potentials of these portions become negative (this phenomenon is called charge-up effect). When the time has come that a positive potential is applied to the metal electrode so as to turn it on and pass electrons through insulating plate 8, the negative potential due to the electrons which have been attached to those portions greatly obstructs the passage of the electrons, thereby lowering the display more than the desired luminance.